The organic light emitting display devices have the property of self-luminescence, and adopt very thin coatings of organic material and glass substrates. The organic material may emit light when current passes therethrough. Moreover, the organic light emitting display devices have display screens with large angle of visibility and can evidently save electric energy, therefore the organic light emitting display devices have superiority over many liquid crystal display devices.
The organic light emitting display devices may be classified into passive matrix types and active matrix types. In the organic light emitting display devices of passive matrix types, pixels are arranged in the matrix form at positions where scan lines and signal lines intersect each other; and in the organic light emitting display devices of active matrix types, each pixel is controlled by a thin film transistor operating as a switch.
FIG. 1 is a circuit diagram showing a pixel circuit of a conventional organic light emitting display device.
Referring to FIG. 1, the pixel circuit of the conventional organic light emitting display device includes a plurality of scan lines G1 to Gn extending in the same direction, a plurality of data lines S1 to Sm extending in the same direction, a plurality of common power lines D1 to Dm extending in the same direction and a plurality of pixel units 101. The number of the data lines is equal to the number of the common power lines. The plurality of data lines S1 to Sm and the plurality of scan line G1 to Gn intersect and are insulated from each other. The plurality of common power lines D1 to Dm and the plurality of scan line G1 to Gn intersect and are insulated from each other. Each pixel unit 101 is defined by a region surrounded by the scan lines, the data line and the common power line.
The circuit diagram of the pixel unit 101 is shown in FIG. 2. Each pixel unit 101 includes a switching thin film transistor 108, a drive thin film transistor 112, a capacitor 110 and an organic light emitting diode 114. One pixel unit 101 is defined by a region surrounded by the scan lines 102, the data line 104 and the common power line 106.
The organic light emitting diode 114 includes a pixel electrode, an organic emitting layer formed on the pixel electrode, and a common electrode formed on the organic emitting layer. The pixel electrode functions as an anode of a hole injecting electrode, and the common electrode functions as a cathode of an electron injecting electrode. In one modification, according to the driving method of the organic light emitting display device, the pixel electrode may be the cathode, and the common electrode may be the anode. Holes and electrons are injected to the organic emitting layer respectively from the pixel electrode and the common electrode to form excitons. When the exciton is changed from an excited state to a ground state, it may emit light.
The switching thin film transistor 108 includes a switching semiconductor layer (not shown in the drawings), a switching gate electrode 107, a switching source electrode 103 and a switching drain electrode 105. The drive thin film transistor 112 includes a driving semiconductor layer (not shown in the drawings), a driving gate electrode 115, a driving source electrode 113 and a driving drain electrode 117.
The capacitor 110 includes a first sustaining electrode 109 and a second sustaining electrode 111, with an interlayer insulating layer interposed between the first sustaining electrode 109 and the second sustaining electrode 111.
The switching thin film transistor 108 functions as a switch for selecting pixels to emit light. The switching gate electrode 107 is connected to the scan line 102. The switching source electrode 103 is connected to the data line 104. The switching drain electrode 105 is provided to be separated from the switching source electrode 103 by a certain distance. The switching drain electrode 105 is connected to the first sustaining electrode 109.
The drive thin film transistor 112 applies drive power to the pixel electrode, such that the organic emitting layer of the organic light emitting diode 114 of the selected pixel emits light. The driving gate electrode 115 is connected to the first sustaining electrode. The driving source electrode 113 and the second sustaining electrode 111 are respectively connected to the common power line 106. The driving drain electrode 117 is connected to the pixel electrode of the organic light emitting diode 114 through a contact hole.
With the above-described structure, the switching thin film transistor 108 is driven by the gate voltage applied to the scan line 102, such that the data voltage applied to the data line 104 is transmitted to the drive thin film transistor 112. A voltage corresponding to a voltage difference between the common voltage transmitted to the drive thin film transistor 112 from the common power line 106 and the data voltage transmitted by the switching thin film transistor 108 is stored in the capacitor 110, and a current corresponding to the voltage stored in the capacitor 110 flows to the organic light emitting diode 114 through the drive thin film transistor 112, thereby the organic light emitting diode 114 emits light.
Further, the voltage source of the organic light emitting display device is a main cause of brightness, therefore the stability of the voltage source is an important index for properties of the organic light emitting display device.
The organic light emitting display devices with high resolutions have become the inevitable trend at present. However, panels with high resolutions have the problems that the charging time becomes short and the number of the data lines increases. Both of the problems may cause the voltage source of the organic light emitting display device to be disturbed and thus can not restore the initial stable potential.
Specifically, in the active matrix organic light emitting display device, brightness is determined by a current passing through the organic light emitting diode. In order to maintain the brightness of the organic light emitting display device to be uniform, the current of the organic light emitting diode needs to be controlled within a range of ±1%. However, the conventional IC circuits all transmit voltage signals instead of current signals, therefore it is difficult for the pixels in the active matrix organic light emitting display devices to accomplish transferring the voltage signals into the current signals within one frame period while keeping respective pixels stable and uniform. The threshold voltage of the drive thin film transistor in the organic light emitting diode drive circuit is one of the important factors for the current.